1. Field of the Invention
The present invention relates to an image processing apparatus for binarizing image data input to the apparatus.
2. Description of the Related Art
In an image scanning apparatus provided in a conventional apparatus such as facsimile machine (hereinafter simply referred to as fax machine) and copy machine, when image data are generated by scanning and digitizing an image consisting of texts and graphics on an original, the image data are subjected to an abnormality determination. In the abnormality determination, whether the image data are usable data is determined by determining whether the digitized image data are all-white data or all-black data.
Usually, the scanning (reading) of an original is performed by a CCD (charge coupled device) image sensor or a CIS (contact image sensor) and resulting image data are a set of data (hereinafter referred to as “pixel data”) representing pieces of gradation information (i.e., pieces of numerical information representing gradations, that is, luminance levels) of an original for respective pixels. The abnormality determination on image data produced by reading is performed on the pieces of gradation information of the respective pixel data. For example, in JP-B-7-105856, whether image data are all-white data or all-black data is determined by converting the gradation information of each pixel data into 1-bit data indicating white or black by using a threshold value (what is called binarization) and determining a ratio between the number of white data and the number of black data.
However, determining whether image data are all-white data or all-black data after binarizing the pixel data using a threshold value has the following problem. For example, where a text and graphics are written or drawn on a gray sheet, if pieces of gradation information corresponding to the ground of an original and a text and graphics on the original are converted into black pixels by using a threshold value, the original is determined all black.
Further, depending on the binarization method, a halftone such as gray may be converted into black pixels and white pixels on a pixel data basis randomly or according to a prescribed calculation method. For example, where no text or the like is formed en a gray sheet, image data of such an original may be determined a mixture of white data and black data, in which case the original read is hardly determined abnormal.